This invention relates to a ceramic grain-interface barrier-layer capacitor and more particularly to a method of making such an intergranular-barrier capacitor wherein strontium titanate is doped with a donor material and manganese, sintered in an inert gas, and then heated to simultaneously oxidize the grain boundary and fire the electrodes.
The ceramic body described in the above-identified cross-referenced patent application, Ser. No. 261,065, is based upon a strontium titanate that is doped with a donor material in such a way that there is created within the ceramic crystal a number of strontium vacancies. Furthermore, the stoichiometry including the amount of donor atoms is adjusted to achieve a balance between large and small cations while the strontium vacancies are depended upon for charge balance in the grains. Such materials, being fired to maturity in air, contain semiconducting grains. They are subsequently impregnated with an oxidizing agent and heated to effect oxidation of the intergranular barrier layers. A silver paste is then applied and the ceramic is heated a third time to fire the silver electrodes. A triple heating process is also used to make the earlier more traditional barrier layer capacitors that are sintered in an active reducing gas.
Excellent capacitors can be made in this manner when employing high purity strontium titanate in the start materials, whereas certain industrial grade strontium titanates, that are lower in cost and more readily available, often contain sufficient quantities and kinds of contaminates such as silicon and a variety of acceptors, that lead to poor quality capacitors with relatively low values of the effective (apparent) dielectric constant.
It would be an improvement of great practical value to the manufacture of disc capacitors and the like if industrial grade start materials could be used and the oxidation could be accomplished simultaneously with the firing on of the silver electrodes. The difficulty with a single oxidizing/electroding heating step has been in oxidizing effectively the semiconducting donor-doped strontium titanate at a temperature below the melting point of silver (960.degree. C.).
Such low temperature oxidation steps tend only to oxidize the grains near the ceramic body surfaces. Such superficial oxidation leads to a hybrid capacitor, between a surface-barrier type and an intergranular-barrier type. As has been described in the above-identified application and further in a paper by Rolf Wernicke presented at the third International Joint Meeting "Electrical and Magnetic Ceramics" on Nov. 17 and 18, 1977 at Noordwijkerhout, the Netherlands, such a capacitor has a few intergranular dielectric barrier layers at the surface, the ceramic material in between consisting of a conducting mass of semiconductive grains. The apparent dielectric constant can be extremely high while the breakdown voltage will be very low. Also such hydrid type capacitors are distinguished by having an apparent dielectric constant that is a strong function of body thickness, while for a true intergranular barrier layer capacitor the apparent dielectric constant is the same for all body thicknesses.
It is an object of this invention to provide a simple and low cost method capable of employing industrial grade start materials for making an intergranular barrier layer capacitor having an apparent dielectric constant that is not a function of body thickness.
It is another object of this invention to provide such a method including sintering in an atmosphere free of active reducing gas.
It is another object of this invention to provide such a method including only two heating steps i.e. one for sintering and another for both oxidizing the grain boundaries and for firing on a silver electrode.
It is a further object of this invention to provide an intergranular boundary layer capacitor with effective dielectric constant greater than 50,000 by using SrTiO.sub.3 of reagent or higher purity.